https://github.com/FreeRDP/Remmina/issues/171
--- a/remmina/CMakeLists.txt
+++ b/remmina/CMakeLists.txt
@@ -148,7 +148,6 @@ if(WITH_TRANSLATIONS)
 endif()
 add_subdirectory(icons)
 add_subdirectory(desktop)
-add_subdirectory(external_tools)
 
 install(TARGETS remmina DESTINATION ${CMAKE_INSTALL_BINDIR})
 
